An optical modulator is a device that converts electrical signals to optical signals for various applications. Currently, the most widely used optical modulators are the electro-optic lithium niobate (LiNbO3) waveguide modulators using traveling-wave (TW) transmission line electrode structures. The structures are designed to achieve velocity-matching conditions of the modulating radio frequency (RF) signals and the optical waves co-propagating down the waveguide circuits to achieve maximum electro-optic interaction. One of the most important optical modulator performance parameter is the electro-optic conversion efficiency.
It has long been universally assumed that a TW transmission line electrode should be designed so that the characteristic impedance is matched as closely as possible to the source drive circuit's transmission line (typically 50Ω) to maximize the electrical signal power transfer to the TW electrode. That design is used for all wide-bandwidth modulators fabricated on any electro-optic material, including LiNbO3, other ferroelectric material, polymer, semiconductor, etc. There are ample works on traveling-wave electro-optic modulators for the last 20-30 years, and their design and operation are widely published in literature.
Optical modulators and switches and all electro-optical devices can be used to electro-optically modulate and switch optical signals. Those optical devices are generally used in fiber-optic systems, optical communications, optical sensors, etc. for both digital and analog applications. Current optical waveguide modulators/switches are based on electro-optic effect in Lithium niobate, other ferroelectric material, polymers, semiconductors, etc. Those devices are commercially available and are being used in optical systems such as broadband digital fiber-optic communications networks, RF analog fiber-optic links, optical sensing, etc.
Optical modulators with conventional designs are being offered by various companies including EOSPACE, JDS Uniphase, Avanex, Sumitomo Cements, Fujitsu, Lumera, Plotline, etc.
An Optical modulator is the key optical component that converts electrical signals into optical signals. An optical switch is used to switch the light signal to different paths. There is always a need to improve the electro-optic conversion/switching efficiency of these devices to improve sensitivity of the optical systems utilizing them.
Current optical modulators are not yet efficient enough (i.e. high drive voltage) to be used for a variety of more demanding applications. Current wideband RF analog fiber-optic links are not sensitive enough for many critical applications, such as those required by advanced fiber-optic antenna remoting for military sensing and surveillance systems. The performances of those RF fiber-optic links are primarily limited by high noise figures and limited sensitivity. Optical modulators with much higher electrical to optical conversion efficiency (i.e. drive voltage) are needed to improve those fiber-optic links' performance.
Broadband optical modulators for digital high-bit-rate fiber-optic systems require low drive voltage so that a lower cost large bandwidth drive amplifier, with lower power consumption, size, and weight, can be used.
In order to achieve both velocity-matching and 50Ω impedance-matching conditions for a traveling-wave optical modulator, it is typically required to make various design and fabrication compromises, resulting in non-optimum efficiency and bandwidth. For example, with the most widely used LiNbO3 TW modulator, the LiNbO3 material itself has very high dielectric constants (∈11˜44, ∈33˜28). That problem not only results in a slow RF signal propagation speed, as compared to that of the light wave signals, but also results in a low transmission line impedance (typically well below 50Ω). That often requires trade-offs with device efficiency, RF bandwidth, etc.
Needs continue to exist for improved electro-optical modulators.